High refractive index contrast photonic integrated circuits (PICs) have applications for use as transceivers for telecommunications. PICs are typically made using platforms such as silicon or indium phosphide. Challenges exist in terms of coupling light efficiently into and out of the PICs with acceptable alignment tolerances, broad bandwidth and low polarization dependence. The high refractive index contrast of these platforms tends to result in relatively small mode field diameters compared with optical fibers which deliver light into and from the platform. The conversion efficiency between dissimilar mode sizes affects the overall coupling performance.
Two possible approaches to providing the optical coupling include grating couplers and edge couplers. Grating couplers comprise a periodic structure which causes an optical signal to be diffracted at an angle relative to the direction of the incident optical signal. Grating couplers produce similar mode sizes to optical fibers. However, grating couplers suffer from poor insertion losses, have a narrow spectral bandwidth and are restrictive in terms of the polarization of the optical signal.
Edge couplers employ spot size converter structures fabricated on top of the platform to allow the mode size to be expanded to a suitable size for coupling into an optical fiber. Edge couplers have low alignment tolerances and typically require the use of a lensed fiber to minimize losses.
This background serves only to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the invention may or may not address one or more of the background issues.